A bit better, much to be done. Maybe do both operations in one traverse, fix the imperative checks and 0 checks. But enough for the day

MissingPositive2.scala

object MissingPositive extends App {

  def missingPositive(nums: Array[Int]): Int = {
    val found: Map[Int, Int] = nums.foldLeft(Map[Int, Int]())((m, i) => if (i >= 0) m + (i -> 1) else m)
    val min = nums
      .filter(e => e > 0)
      .foldLeft(Integer.MAX_VALUE)((m, i) => i.min(m))

    if (min >= 0) {
      var count = 1
      while (found.contains(count)) {
        count += 1
      }

      count
    } else {
      min - 1
    }
  }

  println(missingPositive(Array(1, 2, 0)))
  println(missingPositive(Array(3, 4, -1, 1)))
  println(missingPositive(Array(7, 8, 9, 11, 12)))
}